Positive electrode active material for lithium secondary battery and preparation method thereof

ABSTRACT

Exemplary embodiments of positive electrode active materials in the form of single particles, and a method of preparing each of them, are provided.The single particles of the exemplary embodiments includesingle particles of a nickel-based lithium composite metal oxide, havinga plurality of crystal grains, each having a size of 180 nm to 300 nm, as analyzed by a Cu Kα X-ray (X-rα).The single particles include a metal doped in the crystal lattice thereof. One embodiment includes a surface coating.The total content of the metal doped in the crystal lattice thereof and the metal of the metal oxide coated on the surface thereof is controlled in the range of 2500 ppm to 6000 ppm.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a national stage entry under 35 U.S.C. § 371of PCT/KR2018/014367 filed on Nov. 21, 2018, which claims priority toKorean Patent Application Nos. 10-2017-0156745 and 10-2018-0143840,filed on Nov. 22, 2017 and Nov. 20, 2018, respectively, the disclosuresof which are hereby incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a positive electrode active materialfor a lithium secondary battery having improved lifetime characteristicsat a high voltage, and a preparation method thereof.

BACKGROUND ART

With increasing technology development and demands for mobile devices,demand for secondary batteries as an energy source has rapidlyincreased. Among such secondary batteries, lithium secondary batterieshaving high energy density and voltage, a long cycle life, and a lowself-discharge rate have been commercialized and widely used.

Due to a recent price jump of cobalt (Co), many attempts have been madeto increase price competitiveness by substituting inexpensive lithiumnickel cobalt manganese oxide (NCM) for lithium cobalt oxide (LCO) whichis currently used as a positive electrode active material for smallbatteries.

In the case of NCM, an upper voltage limit in commercial cells is 4.2 V.When NCM with an upper voltage limit of 4.35 V is used in a smalldevice, there is a problem in that it has inferior performance, ascompared to LCO. This is because Li deintercalation of NCM is higherthan that of LCO at the same upper voltage limit, and NCM becomes moreunstable in a high state of charge (SOC). As a result, rapiddeterioration of the lifetime characteristic occurs when NCM is used ata high voltage of 4.35 V or more.

Accordingly, it is necessary to develop NCM positive electrode activematerials having improved lifetime characteristics as an inexpensivepositive electrode active material that is operable at a high voltage.

DISCLOSURE Technical Problem

To solve the above problem, an object of the present invention is toprovide a positive electrode active material for a lithium secondarybattery having an improved lifetime characteristic at a high voltage,and a preparation method thereof.

Technical Solution

An embodiment of the present invention provides a nickel-based lithiumcomposite metal oxide single particle including a plurality of crystalgrains.

Specifically, the single particle may include a metal (one or moremetals selected from the group consisting of M=Al, Ti, Mg, Zr, W, Y, Sr,Co, F, Si, Na, Cu, Fe, Ca, S, and B) doped in a crystal lattice of thesingle particle; or a metal (one or more metals selected from the groupconsisting of M′=Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S,and B) compound coated on the surface of the single particle, togetherwith the metal (one or more metals selected from the group consisting ofM=Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S, and B) dopedin the crystal lattice of the single particle, wherein a total weight ofthe metal doped or the metal in the surface-coated metal compound is2500 to 6000 ppm, and each of the crystal grains included in the singleparticle has a size of 180 nm to 300 nm, as measured by Cu Kα X ray(X-rα).

Another embodiment of the present invention provides a method ofpreparing a positive electrode active material for a lithium secondarybattery, the method including the steps of: preparing a first mixtureincluding a nickel-based lithium composite metal hydroxide particlehaving D50 of 8 μm or less, a lithium raw material, and a metal (one ormore metals selected from the group consisting of M=Al, Ti, Mg, Zr, W,Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S, and B) compound, wherein a contentof the metal (M) compound in the total weight is 2500 ppm to 6000 ppm;and calcining the first mixture at a temperature of 960° C. or higher.

The method may further include the steps of: preparing a second mixtureincluding the calcined product of the first mixture, and a metal (one ormore metals selected from the group consisting of M′=Al, Ti, Mg, Zr, W,Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S, and B) compound; and calcining thesecond mixture at a temperature of 350° C. to 800° C., after the step ofcalcining the first mixture at a temperature of 960° C. or higher.

In this regard, the coating element M′ and the doping element M may bethe same as or different from each other.

Still another embodiment of the present invention provides a positiveelectrode for a lithium secondary battery, the positive electrodeincluding the above-described positive electrode active material, and alithium secondary battery including the positive electrode.

Effect of the Invention

A positive electrode active material for a lithium secondary batteryaccording to the present invention may improve the lifetimecharacteristic at a high voltage.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1F show scanning electron microscopy (SEM) images ofpositive electrode active materials prepared in Examples 2, 4, 6, 8, 10,and 12, respectively;

FIGS. 2A to 2G show images of TEM-ASTAR and HR-TEM of Example 2 andComparative Example 1;

FIGS. 3A and 3B show gas generation of positive electrode activematerials according to examples and comparative examples; and

FIGS. 4A and 4B show graphs of comparing lifetime characteristicsaccording to Experimental Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a method of preparing a positive electrode active materialfor a lithium secondary battery, a positive electrode active materialprepared by the method, a positive electrode including the positiveelectrode active material, and a lithium secondary battery according tospecific embodiments of the present invention will be described.

In the present specification and claims, “including a plurality ofcrystal grains” means a crystal formed by two or more crystal particleshaving an average crystallite size in a particular range. In thisregard, the crystallite size of the crystal grain may be quantitativelyanalyzed by X-ray diffraction (XRD) analysis with Cu Kα X-rays (X-rα).Specifically, the prepared particles are put in a holder, anddiffraction grating obtained by irradiating X-rays onto the particles isanalyzed, thereby quantitatively analyzing the average crystallite sizeof the crystal grain.

Further, in the present specification and claims, D50 may be defined asa particle size at 50% of a particle size distribution, and may bemeasured by using a laser diffraction method.

In addition, it will be understood that terms or words used in thespecification and claims shall not be interpreted as the meaning definedin commonly used dictionaries, and the words or terms should beinterpreted as having a meaning that is consistent with their meaning inthe context of the relevant art and the technical idea of the invention,based on the principle that an inventor may properly define the meaningof the words or terms to best explain the invention.

Positive Electrode Active Material

In the present invention, two exemplary embodiments of a positiveelectrode active material in the form of single particle are provided.

Specifically, single particles of two exemplary embodiments may commonlyinclude:

1) a single particle of a nickel-based lithium composite metal oxide;

2) a plurality of crystal grains, each having a size of 180 nm to 300nm, as analyzed by X-ray diffraction (XRD) analysis with a Cu Kα X-ray(X-rα); and

3) a metal doped in the crystal lattice (one or more metals selectedfrom the group consisting of M=Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na,Cu, Fe, Ca, S, and B).

4) However, the single particles of two exemplary embodiments may bedistinguished from each other by surface coating.

4-1) More specifically, the single particle of one exemplary embodiment(hereinafter referred to as a ‘first single particle’) includes themetal doped in the crystal lattice thereof (one or more metals selectedfrom the group consisting of M=Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na,Cu, Fe, Ca, S, and B), but the surface thereof is not coated.

4-2) The single particle of another exemplary embodiment (hereinafterreferred to as a ‘second single particle’) includes the metal (one ormore metals selected from the group consisting of M=Al, Ti, Mg, Zr, W,Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S, and B) doped in the crystal latticethereof and a metal oxide (of one or more metals selected from the groupconsisting of M′=Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S,and B) coated on the surface thereof.

4-3) However, in the first and second single particles, a total contentof the metal doped in the crystal lattice thereof, and the metal of themetal oxide coated on the surface thereof, is controlled in the range of2500 ppm to 6000 ppm. That is, since the first single particle does notinclude a metal oxide coated on the surface thereof, the content of themetal doped in the crystal lattice thereof is 2500 ppm to 6000 ppm.Further, in the second single particle, the total content of the metaldoped in the crystal lattice thereof, and the metal of the metal oxidecoated on the surface thereof, is 2500 ppm to 6000 ppm.

The first and second single particles may have the characteristics of 1)to 4), thereby contributing to improvement of lifetime characteristicsof a battery at a high voltage.

Hereinafter, the characteristics of 1) to 4) of the first and secondsingle particles will be described in detail.

Shape of Particle and Size of Crystal Grain

1) Generally, a nickel-based lithium composite metal oxide is asecondary particle, but the first and second single particles are anickel-based lithium composite metal oxide embodied in the form ofsingle particle.

Specifically, a nickel-based lithium composite metal hydroxide secondaryparticle prepared by co-precipitation is used as a precursor, thisprecursor is mixed with a lithium raw material, and this mixture iscalcined at a temperature of lower than 960° C. to obtain a nickel-basedlithium composite metal oxide secondary particle.

However, when the calcination temperature is increased to 960° C. orhigher, the particle no longer has a secondary particle form, and singleparticles of the nickel-based lithium composite metal oxide may beobtained. This resulting product may be used as the single particles ofthe first and second embodiments.

2) Meanwhile, the nickel-based lithium composite metal oxide secondaryparticle and the single particles of the two embodiments include aplurality of crystal grains, regardless of the calcination temperature.Here, “including a plurality of crystal grains” means a crystal formedby two or more crystal particles having an average crystallite size in aparticular range. In this regard, the crystallite size of the crystalgrain may be quantitatively analyzed by X-ray diffraction (XRD) analysiswith a Cu Kα X-ray (X-rα). Specifically, the prepared particles are putin a holder, and diffraction grating obtained by irradiating X-rays ontothe particles is analyzed, thereby quantitatively analyzing the averagecrystallite size of the crystal grain.

However, depending on the calcination temperature, a difference mayoccur in the crystal grain size. Specifically, when calcination isperformed at a low temperature of lower than 960° C., crystal grainshaving a size of less than 180 nm may be produced. That is, when thesize of crystal grains is less than 180 nm, it is difficult to obtain asingle particle having a perfect shape. As a result, there is a concernthat the interface between a positive electrode active material and anelectrolyte may become large, and contact between the particles and theelectrolyte may be increased due to a volume change duringcharge/discharge. Further, when the average crystallite size of theparticle exceeds 300 nm, there is a concern that the capacity may bereduced due to enlargement of the crystal grains.

In contrast, when the single particles of the first and secondembodiments are excessively calcined at a temperature of 960° C. orhigher, specifically at 960° C. to 1100° C., for example at 990° C. to1100° C., coarsening of crystal grains may occur, and therefore singleparticles having a size of up to 180 nm to 300 nm may be included.

As such, the first and second single particles including the coarsenedcrystal grains (180 nm to 300 nm) have a lower specific surface area(BET) than the nickel-based lithium composite metal oxide secondaryparticle including smaller crystal grains, and therefore its reactivitywith an electrolyte is low, and gas generation and release of internalmetals may be reduced.

Generally, as the calcination temperature increases, the size of thesingle particle and the size of the crystal grain in the single particleincrease at the same composition, regardless of the particle shape.However, the results of X-ray diffraction analysis with a Cu Kα X-ray(X-rα) shows that the crystal grains in the single particles of the twoembodiments have a larger size when they are over-calcined at atemperature of 990° C. (Examples 1 to 6) than those over-calcined at atemperature of 1010° C. (Examples 7 to 12) with the same composition.This is because the patterns resulting from over-calcination at 1010° C.(Examples 7 to 12) are related to an aggregation degree of singleparticles according to the calcination temperature and a measuringdevice, rather than being out of the general tendency.

When single particles are over-calcined at 990° C. (Examples 1 to 6),relatively small single particles may aggregate together and look likesecondary particles. That is, it is understood that XRD recognizes thesecondary particle-like look, and recognizes boundaries between thesingle particles as grain boundaries to detect the single particle sizeas the crystal grain size.

Unlike this, when single particles are over-calcined at 1010° C.(Examples 7 to 12), relatively large single particles may be separatelyobserved while maintaining a distance from each other. Therefore, it isunderstood that XRD recognizes individual single particles, andcorrectly recognizes the crystal grain size in the single particles.

Average Particle Size (D50) and Crystal Structure

There are differences in the average particle size (D50) and the crystalstructure between the nickel-based lithium composite metal oxidesecondary particles and the single particles of the two embodiments.

The average particle size (D50) of the nickel-based lithium compositemetal oxide secondary particles may be 10 μm to 18 μm, and the averageparticle size (D50) of the single particles of the two embodiments maybe 3.5 μm or more to 8 μm or less.

As D50 of particles decreases, a BET specific surface area of an activematerial increases. However, BET reduction due to the increase of thecrystal grain size caused by the over-calcination is predominant, ascompared to BET increase due to reduction of the particle size. As aresult, the interface between an electrolyte and a positive electrodeactive material may be minimized to reduce side reactions and to improvebattery performance. Further, by preparing the active material as smallparticles, capacity and efficiency may be increased and single particlesmay be more easily prepared.

When the average particle size (D50) of the single particles of the twoembodiments is less than 3.5 μm, there are concerns about reduction indurability of the electrolyte and the positive electrode activematerial. When the average particle size (D50) of the single particlesof the two embodiments is more than 8 μm, there are concerns aboutcapacity reduction due to over-calcination and reduction of lifetimecharacteristics. Therefore, the average particle size (D50) of thesingle particles of the two embodiments may be controlled in the rangeof 3.5 μm or more to 8 μm or less.

Meanwhile, the crystal lattice structure of the nickel-based lithiumcomposite metal oxide secondary particles may have a layered structurethroughout the interior thereof. This is attributed to uniform formationof a layered structure of lithium (Li) layer—cation mixing layer—lithium(Li) layer—cation mixing layer from the surface of the particle towardthe center of the particle at a temperature of less than 960° C.

In contrast, the single particles of the two embodiments may havedifferent crystal structures at the central part thereof and the surfacepart thereof. This is attributed to the structural change of theparticle surface from the layered structure to a rock salt structure, aspinel structure, or a mixed structure thereof at a temperature of 960°C. or higher. Specifically, at a temperature of 960° C. or higher, 960°C. to 1100° C., or for example 990° C. to 1100° C., loss of part oflithium occurs on the surface of the nickel-based lithium compositemetal oxide single particles, and Ni²⁺ ions of the lower part thereofmove to the underlying lithium (Li) layer, that is, degeneration of thecation mixing layer successively occurs. These successive reactions mayoccur from the surface of the nickel-based lithium composite metal oxidesingle particles to a particular depth.

Specifically, the single particles of the two embodiments may have asurface part and a central part which are different from each other inthe crystal lattice structure.

More specifically, the surface part of the single particles of the twoembodiments may include a rock salt structure, a spinel structure, or amixed structure thereof from the surface of the individual singleparticle to a depth of 0.13% to 5.26%, for example of a depth of 0.13%to 2.63% of the radius of the individual single particle. Meanwhile, thecentral part of the single particles of the two embodiments may have alayered structure from the interface with the surface part to the centerof the single particle.

The surface part of the rock salt structure, the spinel structure, orthe mixed structure thereof of the single particles of the twoembodiments may have low reactivity with an electrolyte, may bestructurally stable, and may also maintain the structural stability evenif a charge voltage of more than 4.2 V, 4.4 V or more, or 4.45 V or moreis applied, as compared with the surface part of the layered structureof the nickel-based lithium composite metal oxide secondary particles.

Composition

3) The first single particle may include a metal (M) doped in thecrystal structure of a lithium composite metal oxide represented by thefollowing Chemical Formula 1; and the second single particle may includea metal compound coated on the surface of the single particle, togetherwith the metal (M) doped in the crystal structure of lithium compositemetal oxide represented by the following Chemical Formula 1:Li_(a)(Ni_(x)Mn_(y)Co_(z))O_(2+b)  (1)

wherein, in Chemical Formula 1, a, x, y, and z represent a molar ratioof each element in the lithium composite metal oxide.

The metal (M) doped in the crystal lattice of the first and secondsingle particles and the metal (M′) of the metal compound coated on thesurface of the second single particle may be the same as or differentfrom each other, and each may be one or more metals selected from thegroup consisting of Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na, Cu, Fe, Ca,S, and B.

In the first and second single particles, the element (M) may be locatedonly on part of the surface of the particle depending on the positionalpreference of the element M, and may be located with a concentrationgradient from the surface of the particle toward the center of theparticle, or may uniformly exist throughout the particle.

When the first and second single particles are doped, or coated anddoped, with the same element (M), high voltage characteristics of theactive material may be further improved due to stabilization of thesurface structure. Among the elements, the element (M), specifically,three elements of Ti, Mg, and Zr, may be doped, or coated and doped, orZr alone may be doped, or coated and doped, in terms of excellentsurface structure-stabilizing effects.

The element (M) may be doped, or coated and doped, in an amount of 2500ppm to 6000 ppm. When the amount of the element (M) is less than 2500ppm, the structure-stabilizing effect according to doping or coating ofthe element (M) may be slight, and when the amount of the element (M) ismore than 6000 ppm, there are concerns about capacity reduction anddeterioration of lifetime characteristics and rate characteristics dueto increased resistance. The element (M) may be coated or doped in aspecific amount of 2500 to 5500 ppm, and more specifically, 3500 ppm to5000 ppm, in terms of an excellent improvement effect by control of theelement (M) content.

Meanwhile, Chemical Formula 1 may follow a composition of a generallyknown nickel-based lithium composite metal oxide secondary particle.

For example, in Chemical Formula 1, lithium (Li) may be included in anamount corresponding to a, specifically 0.95≤a≤1.2. When a is less than0.95, there are concerns about deterioration of output characteristicsof a battery due to an increase of interface resistance generated at thecontact interface between the positive electrode active material and theelectrolyte. On the contrary, when a is more than 1.2, there areconcerns about reduction of an initial discharge capacity of a battery.In terms of the effects of combination with the characteristic structureof the active material according to one embodiment of the presentinvention, a may more specifically satisfy 0.98≤a<1.07, and much morespecifically a=1.

In Chemical Formula 1, nickel (Ni) is an element that contributes tohigh voltage and high capacity of a secondary battery. The nickel (Ni)may be included in an amount corresponding to x, specifically, 0<x<0.6,under conditions satisfying x+y+z=1. When x is 0, there are concernsabout deterioration of charge/discharge capacity characteristics. When xis 0.6 or more, there are concerns about deterioration of structural andthermal stability of the active material, and as a result, deteriorationof lifetime characteristics. In terms of high voltage and high capacityeffects by control of the nickel content, the nickel may be specificallyincluded in an amount of 0.4≤x<0.6, and more specifically 0.5≤x<0.6,under conditions satisfying x+y+z=1.

Further, in Chemical Formula 1, manganese (Mn) is an element thatcontributes to improvement of thermal stability of the active material,and it may be included in an amount corresponding to y, specifically0≤y≤0.4, under conditions satisfying x+y+z=1. When y is more than 0.4,there are concerns about deterioration of lifetime characteristics dueto increased release of manganese. In terms of improvement of thermalstability of the active material and improvement of lifetimecharacteristics by control of the manganese content, the manganese maymore specifically be included in an amount of 0.1≤y<0.4, and much morespecifically 0.1≤y≤0.3, under conditions satisfying x+y+z=1.

Further, in Chemical Formula 1, cobalt (Co) is an element thatcontributes to improvement of charge/discharge cycle characteristics ofthe active material, and it may be included in an amount correspondingto z, specifically 0≤z<1, under conditions satisfying x+y+z=1. When z is1, there are concerns about deterioration of charge/discharge capacitydue to deterioration of structural stability. In terms of improvement ofcycle characteristics of the active material by control of the cobaltcontent, the cobalt may be more specifically included in an amount of0.1≤z≤0.4, and much more specifically 0.1≤z≤0.3.

That is, Chemical Formula 1 may satisfy a=1, 0≤b≤0.02, 0.4<x<0.6,0.1≤y<0.4, 0.1≤z<0.4, and x+y+z=1, and more specifically,LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂, LiNi_(0.5)Co_(0.3)Mn_(0.2)O₂, or the like.Among them, any one or a mixture of two or more thereof may be includedin the positive electrode active material.

Method of Preparing Positive Electrode Active Material

The present invention provides a method of preparing the singleparticles of the two embodiments.

Specifically, the first single particle may be prepared by a series ofprocesses including the steps of:

preparing a first mixture including a lithium composite metal hydroxideparticle having D50 of 8 μm or less, a lithium raw material, and a metal(one or more selected from the group consisting of M=Al, Ti, Mg, Zr, W,Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S, and B) compound; and

calcining the first mixture at a temperature of 960° C. or higher.

In the step of calcining the first mixture at a temperature of 960° C.or higher, the nickel-based lithium composite metal oxide singleparticle including a plurality of crystal grains may be synthesized, andat the same time, the metal (M) of the metal (M) compound may be dopedin the crystal lattice of the lithium composite metal oxide singleparticle, thereby obtaining the first single particle.

Considering the content of the metal doped in the first single particlefinally obtained, the content of the metal (M) compound in the totalweight of the first mixture may be controlled in the range of 2500 ppmto 6000 ppm.

Meanwhile, the second single particle may be prepared by a series ofprocesses including the steps of:

preparing a first mixture including a nickel-based lithium compositemetal hydroxide particle having D50 of 8 μm or less, a lithium rawmaterial, and a metal (one or more selected from the group consisting ofM=Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S, and B)compound;

calcining the first mixture at a temperature of 960° C. or higher;

preparing a second mixture including a calcined product of the firstmixture and a metal (one or more selected from the group consisting ofM′=Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S, and B)compound; and

calcining the second mixture at a temperature of 350° C. to 800° C.

Like in the preparation of the first single particle, in the step ofcalcining the first mixture at a temperature of 960° C. or higher duringthe preparation of the second single particle, the nickel-based lithiumcomposite metal oxide single particle including a plurality of crystalgrains may be synthesized, and at the same time, the metal (M) of themetal (M) compound may be doped in the crystal lattice of the lithiumcomposite metal oxide single particle.

However, unlike the preparation of the first single particle, the stepof calcining the second mixture at a temperature of 350° C. to 800° C.may be further performed after preparing the second mixture includingthe calcined product of the first mixture and the metal (M′) compound,thereby obtaining the second single particle.

Further, the total content of the metal (M) compound and the metal (M′)compound in the total weight of the first mixture and the second mixturemay be 2500 ppm to 6000 ppm.

With regard to the method of preparing the first or second singleparticle, those overlapped with the above-described explanations will beexcluded, and process features will be described in detail.

Calcination Process

During the preparation of the first and second single particles, whenthe first mixture, that is, the mixture of the lithium composite metalhydroxide particle having D50 of 8 μm or less, the lithium raw material,and the metal (M) compound is calcined at a temperature of 960° C. orhigher, the lithium composite metal oxide single particle including aplurality of crystal grains may be synthesized, and at the same time,the metal (M) of the metal (M) compound may be doped in the crystallattice of the lithium composite metal oxide single particle.

The calcination temperature may be controlled to a temperature of 960°C. or higher, considering the above-described factors such as singleparticle preparation, size control of crystal grains, etc. Morespecifically, the calcination temperature may be controlled at atemperature of 960° C. to 1100° C., for example 990° C. to 1100° C. Inparticular, when the composition of Ni is less than 0.6,over-calcination may not occur at a temperature of lower than 960° C.Therefore, when the calcination temperature is lower than 960° C., D50of the active material to be prepared is not as large as the rangeaccording to the present invention, and thus there are concerns aboutdeterioration of battery performance due to side reactions between theelectrolyte and the positive electrode active material at a high SOC. Onthe contrary, when the calcination temperature is higher than 1100° C.,it is not preferable in that the capacity may be reduced due toenlargement of the crystal grains.

The over-calcination process may be performed in an oxidizing atmospherecontaining oxygen, and more specifically, in an atmosphere having anoxygen content of 20 vol % or more.

The lithium composite metal hydroxide particle having D50 of 8 μm orless may be a precursor for the preparation of the above-describedsingle particle of one embodiment, which is represented by the followingChemical Formula 2:(Ni_(x)Mn_(y)Co_(z))OH_(2+b)  (2)

wherein, in Chemical Formula 2, 0.95≤a≤1.2, 0≤b≤0.02, 0<x<0.6, 0≤y≤0.4,0≤z<1, and x+y+z+=1.

The average particle size (D50) of the lithium composite metal hydroxideparticle may more specifically be 4 μm or more to 8 μm or less. When thepositive electrode active material is prepared using the precursorhaving a size in the above range, it is possible to prepare a positiveelectrode active material having D50 of 4 μm or more to 8 μm or less inthe form of a single particle.

Specifically, the lithium composite metal hydroxide particle having D50of 8 μm or less as a precursor may be an oxide, hydroxide, oroxyhydroxide containing nickel, cobalt, and manganese.

The precursor may be prepared according to a common method, except thatthe raw materials of nickel, cobalt, and manganese are used in amountsdefined in Chemical Formula 1, and the average particle size (D50) ofthe finally prepared precursor is 7 μm or less. For example, theprecursor may be prepared by a solid-phase method of mixing nickeloxide, cobalt oxide, and manganese oxide, and then performing heattreatment, or prepared by a co-precipitation method of adding respectivemetal salts containing nickel, cobalt, and manganese to a solvent,specifically, water or a mixture of water and an organic solvent that ismiscible with water (specifically, an alcohol, etc.).

Further, the lithium raw material may be lithium-containing oxide,sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide,hydroxide, oxyhydroxide, etc., and specifically, Li₂CO₃, LiNO₃, LiNO₂,LiOH, LiOH.H₂O, LiH, LiF, LiCl, LiBr, LiI, CH₃COOLi, Li₂O, Li₂SO₄,CH₃COOLi, Li₃C₆H₅O₇, etc. Any one of them or a mixture of two or morethereof may be used. Among them, the lithium raw material may be Li₂O orLi₂CO₃, in terms of reaction efficiency and reduction of by-productproduction upon reaction with the precursor for forming the lithiumcomposite metal oxide.

Further, the raw material of the doping element M may be an elementM-containing oxide, sulfate, nitrate, acetate, carbonate, oxalate,citrate, halide, hydroxide, oxyhydroxide, etc. In this regard, M is thesame as described above.

The precursor for forming the lithium composite metal oxide, the lithiumraw material, and the raw material of element M or M′ may be used afterbeing mixed in amounts satisfying the content range of the lithium inthe finally prepared lithium composite metal oxide of Chemical Formula 1and the content range of the element M (corresponding to a sum of M andM′ in the preparation method) included in the positive electrode activematerial.

Coating Process

The method of preparing the second single particle may further includethe steps of preparing a second mixture including the calcined productof the first mixture and a metal (one or more metals selected from thegroup consisting of an M′=Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na, Cu,Fe, Ca, S, and B) compound, and calcining the second mixture at atemperature of 350° C. to 800° C., after the step of calcining the firstmixture at a temperature of 960° C. or higher.

When this process is further included, a coating layer containing themetal (M′) compound may be formed on all or part of the surface of thesingle particle, and the metal compound may be, for example, in the formof a metal oxide.

In this regard, the coating element M′ may be an element that is thesame as or different from the doping element M. Further, the rawmaterial of the coating element M′ may be an element M′-containingoxide, sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide,hydroxide, oxyhydroxide, etc. In this regard, M is the same as describedabove. An additional heat treatment process of forming the coating layermay be performed at a temperature of 350° C. to 800° C., morespecifically at a temperature of 350° C. to 650° C., in an oxidizingatmosphere containing oxygen, as in the doping process.

When the secondary heat treatment temperature for additional coating islower than 350° C., the coating layer may not be sufficiently formed,and when the temperature is higher than 800° C., there are problems ofcation mixing and deterioration of capacity and rate characteristics.

Post-Treatment Process

Meanwhile, a cooling process may be optionally performed after theover-calcination during the preparation of the first single particle andafter the secondary heat treatment during the preparation of the secondsingle particle.

The cooling process may be performed according to a common method,specifically, by a method such as natural cooling in an air atmosphereor hot air cooling.

Since the positive electrode active material for a lithium secondarybattery prepared according to the above-described preparation method mayhave a reduced specific surface area (BET), an interface with anelectrolyte may be reduced. In addition, the amount of residual lithiumin the active material is reduced due to over-calcination, and thusside-reaction with an electrolyte may be reduced. Accordingly, thepositive electrode active material may exhibit excellent batteryperformance and lifetime characteristics when the battery is operated ata high voltage of 4.35 V or more, and in particular, it may exhibitexcellent high-temperature lifetime characteristics due to structuralstability.

Lithium Secondary Battery

Still another embodiment of the present invention provides a positiveelectrode for a lithium secondary battery, the positive electrodeincluding the positive electrode active material including any one ofthe single particles of the two embodiments, and a lithium secondarybattery including the positive electrode.

The positive electrode and the lithium secondary battery each preparedby using the above-described positive electrode active material mayexhibit excellent lifetime characteristics even at a high SOC.

Specifically, the positive electrode may include a positive electrodecollector and a positive electrode active material layer which is formedon the positive electrode collector and includes the above-describedpositive electrode active material.

The positive electrode collector is not particularly limited as long asit has conductivity without causing chemical changes in a battery. Forexample, stainless steel, aluminum, nickel, titanium, calcined carbon,and aluminum or stainless steel that is surface-treated with carbon,nickel, titanium, or silver may be used. The positive electrodecollector may have a thickness of 3 μm to 500 μm, and adhesion of thepositive electrode active material may be increased by forming fineroughness on the surface of the collector. For example, the collectormay be in a variety of forms such as a film, a sheet, a foil, a net, aporous material, a foamed material, a non-woven fabric material, etc.

Further, the positive electrode active material layer may include aconductive material and a binder together with the above-describedpositive electrode active material.

In this regard, the conductive material is used to provide an electrodewith conductivity, and is not particularly limited as long as it haselectrical conductivity without causing chemical changes in the battery.Specific examples thereof may include carbon-based materials such ascarbon black, acetylene black, Ketjen black, channel black, furnaceblack, lamp black, thermal black, carbon fibers, etc.; graphite such asnatural or artificial graphite; metallic powders or metallic fibers ofcopper, nickel, aluminum, silver, etc.; conductive whiskers such as zincoxide, potassium titanate, etc.; conductive metal oxides such astitanium oxide; or conductive polymers such as polyphenylenederivatives, etc. These may be used alone or in a mixture of two or morethereof. The conductive material may be included in an amount of 1% byweight to 30% by weight with respect to the total weight of the positiveelectrode active material layer.

Further, the binder functions to improve adhesion between the positiveelectrode active materials, and adhesive force between the positiveelectrode active material and the collector. Specific examples thereofmay include polyvinylidene fluoride (PVDF), a vinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol,polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinyl pyrrolidone,tetrafluoroethylene, polyethylene, polypropylene, anethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrenebutadiene rubber (SBR), fluorine rubber, or combinations thereof. Thesemay be used alone or in a mixture of two or more thereof. The binder maybe included in an amount of 1% by weight to 30% by weight with respectto the total weight of the positive electrode active material layer.

The positive electrode may be manufactured according to a common methodof manufacturing a positive electrode, except that the above-describedpositive electrode active material is used. Specifically, the positiveelectrode may be manufactured by applying a slurry for forming apositive electrode active material layer onto a positive electrodecollector, followed by drying and rolling, in which the slurry isprepared by mixing the above-described positive electrode activematerial and optionally the binder, and the conductive material in asolvent. In this regard, the kind and content of the positive electrodeactive material, the binder, and the conductive material are the same asdescribed above.

The solvent may be a solvent generally used in the art, and may includedimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP),acetone, water, etc. These may be used alone or in a mixture of two ormore thereof. In consideration of the coating thickness of the slurryand the production yield, the solvent may be used in an amountsufficient to dissolve or disperse the positive electrode activematerial, the conductive material, and the binder, and to allow theslurry to have a viscosity suitable for excellent uniformity ofthickness when applied for manufacturing the positive electrode.

As an alternative method, the positive electrode may be manufactured bycasting the slurry for forming the positive electrode active materiallayer on a separate support, and then laminating a film peeled off fromthe support on the positive electrode collector.

Still another embodiment of the present invention provides anelectrochemical device including the positive electrode. Theelectrochemical device may specifically be a battery, a capacitor, etc.,and more specifically, a lithium secondary battery.

The lithium secondary battery may specifically include a positiveelectrode, a negative electrode facing the positive electrode, aseparator interposed between the positive electrode and the negativeelectrode, and an electrolyte. The positive electrode is the same asdescribed above. Also, the lithium secondary battery may optionallyfurther include a battery container accommodating an electrode assemblycomposed of the positive electrode, the negative electrode, and theseparator, and a sealing member for sealing the battery container.

In the lithium secondary battery, the negative electrode includes anegative electrode collector and a negative electrode active materiallayer disposed on the negative electrode collector.

The negative electrode collector is not particularly limited as long asit has high conductivity without causing chemical changes in a battery.For example, copper, stainless steel, aluminum, nickel, titanium,calcined carbon, or copper or stainless steel that is surface-treatedwith carbon, nickel, titanium, or silver, or an aluminum-cadmium alloy,may be used. Also, the negative electrode collector may commonly have athickness of 3 μm to 500 μm, and like the positive electrode collector,adhesion of the negative electrode active material may be increased byforming fine roughness on the surface of the collector. For example, thecollector may be in a variety of forms such as a film, a sheet, a foil,a net, a porous material, a foamed material, a non-woven fabricmaterial, etc.

Further, the negative electrode active material layer may optionallyinclude a binder and a conductive material together with the negativeelectrode active material. The negative electrode active material layermay be prepared, for example, by applying and drying a slurry forforming the negative electrode on the negative electrode collector, theslurry being prepared by mixing the negative electrode active material,and optionally the binder, and the conductive material in a solvent, orby casting a composition for forming the negative electrode on aseparate support, and then laminating a film peeled off from the supporton the negative electrode collector.

The negative electrode active material may be a compound that is capableof reversible intercalation and deintercalation of lithium. Specificexamples thereof may include any one or a mixture of two or more of acarbon material such as artificial graphite, natural graphite,graphitized carbon fiber, amorphous carbon, etc.; a metal compound thatis capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In,Mg, Ga, Cd, a Si alloy, a Sn alloy, an Al alloy, etc.; a metal oxidethat is capable of doping and dedoping lithium ions, such as SiO_(x)(0<x<2), SnO₂, vanadium oxide, or lithium vanadium oxide; and acomposite including the metal compound and the carbon material, such asa Si—C composite or a Sn—C composite. Also, the negative electrodeactive material may be a lithium metal thin film. In addition,low-crystallinity carbon, high-crystallinity carbon, etc. may all beused as the carbon material. The low-crystallinity carbon is representedby soft carbon or hard carbon, and the high-crystallinity carbon isrepresented by high-temperature calcined carbon such as amorphous,platy, flake, spherical, or fibrous natural graphite or artificialgraphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbonfiber, mesocarbon microbeads, mesophase pitches, petroleum or coal tarpitch-derived cokes, or the like.

In addition, the binder and the conductive material may be the same asthose described in the positive electrode.

Meanwhile, in the lithium secondary battery, the separator serves toseparate the negative electrode and the positive electrode and toprovide a flow passage for lithium ions. The separator is notparticularly limited as long as it is used as a separator in a commonlithium secondary battery, and particularly, a separator which exhibitslow resistance to migration of electrolyte ions and has an excellentability of absorbing an electrolyte solution is preferred. Specifically,the separator may be a porous polymer film made of a polyolefin-basedpolymer such as an ethylene homopolymer, a propylene homopolymer, anethylene/butene copolymer, an ethylene/hexene copolymer, anethylene/methacrylate copolymer, or the like, or a stacked structurehaving two or more layers thereof. Alternatively, the separator may be acommon porous non-woven fabric, for example, a non-woven fabric made ofglass fiber with a high melting point, polyethylene terephthalate fiber,or the like. Also, in order to ensure heat resistance or mechanicalstrength, the separator may be a coated separator including ceramiccomponents or polymer materials, and optionally, may be used in asingle-layer or multi-layer structure.

In addition, the electrolyte used in the present invention may be anorganic liquid electrolyte, an inorganic liquid electrolyte, a solidpolymer electrolyte, a gel polymer electrolyte, an inorganic solidelectrolyte, a molten-type inorganic electrolyte, etc., which may beused in the manufacture of a lithium secondary battery, but the presentinvention is not limited thereto.

Specifically, the electrolyte may include an organic solvent and alithium salt.

The organic solvent is not particularly limited as long as it may act asa medium capable of migrating ions involved in an electrochemicalreaction of a battery. Specifically, the organic solvent may be anester-based solvent such as methyl acetate, ethyl acetate,γ-butyrolactone, ε-caprolactone, etc.; an ether-based solvent such asdibutyl ether, tetrahydrofuran, etc.; a ketone-based solvent such ascyclohexanone, etc.; an aromatic hydrocarbon-based solvent such asbenzene, fluorobenzene, etc.; a carbonate-based solvent such as dimethylcarbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC),ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylenecarbonate (PC), etc.; an alcohol-based solvent such as ethyl alcohol,isopropyl alcohol, etc.; nitriles such as R—CN (where R is a C2 to C20hydrocarbon group with a linear, branched, or cyclic structure, and mayinclude a double-bond aromatic ring or an ether linkage), etc.; amidessuch as dimethylformamide, etc.; dioxolanes such as 1,3-dioxolane, etc.;or sulfolane. Among these compounds, the carbonate-based solvent ispreferred, and a mixture of a cyclic carbonate (e.g., ethylenecarbonate, propylene carbonate, etc.) having high ionic conductivity anda high dielectric constant, which is capable of increasing the chargingand discharging performance of a battery, and a linear carbonate-basedcompound with low viscosity (e.g., ethyl methyl carbonate, dimethylcarbonate, diethyl carbonate, etc.) is more preferred. In this case,when the cyclic carbonate and chain carbonate are used after being mixedat a volume ratio of about 1:1 to about 1:9, excellent performance ofthe electrolyte may be exhibited.

The lithium salt is not particularly limited as long as it may providelithium ions used in a lithium secondary battery. Specifically, thelithium salt may be LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiSbF₆, LiAlO₄,LiAlCl₄, LiCF₃SO₃, LiC₄F₉SO₃, LiN(C₂F₅SO₃)₂, LiN(C₂F₅SO₂)₂,LiN(CF₃SO₂)₂, LiCl, LiI, LiB(C₂O₄)₂, etc. The concentration of thelithium salt is preferably within a range of 0.1 M to 2.0 M. When theconcentration of the lithium salt is within the above range, anelectrolyte has appropriate conductivity and viscosity, and thusexcellent performance of the electrolyte may be exhibited and lithiumions may be effectively migrated.

In addition to the electrolyte components, for the purpose of improvinglifetime characteristics of the battery, suppressing a decrease inbattery capacity, improving discharge capacity of the battery, etc., theelectrolyte may further include one or more additives such as ahaloalkylene carbonate-based compound such as difluoroethylenecarbonate, etc., pyridine, triethyl phosphite, triethanolamine, a cyclicether, ethylene diamine, n-glyme, hexaphosphoric triamide, nitrobenzenederivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinone,N,N-substituted imidazolidine, ethylene glycol dialkyl ethers, ammoniumsalts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. In thisregard, the additives may be included in an amount of 0.1% by weight to5% by weight with respect to the total weight of the electrolyte.

The lithium secondary battery including the positive electrode activematerial according to the present invention may stably exhibit excellentdischarge capacity, output characteristics, and capacity retention rate,thereby being usefully applied in portable devices such as mobilephones, notebook PCs, digital cameras, etc., and electric vehicles suchas a hybrid electric vehicles (HEV), etc.

Accordingly, still another embodiment of the present invention providesa battery module including the lithium secondary battery as a unit celland a battery pack including the same.

The battery module or battery pack may be used as a power source of oneor more medium- and large-sized devices such as a power tool, anelectric car including an electric vehicle (EV), a hybrid electricvehicle, and a plug-in hybrid electric vehicle (PHEV), or a powerstorage system.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail so that one of ordinary skill in the art to which the presentinvention pertains can easily practice the present invention. However,the present invention may be implemented in various other forms and isnot limited to the embodiments described herein. Further, methods usedin experimental examples to measure physical properties of activematerials or precursors are the same as described below.

1) Average particle size D50 (μm): Particle sizes at 50% and 10% ofparticle size distribution were measured for active materials orprecursors by a laser diffraction method (Microtrac MT 3000),respectively.

2) Crystallite size (nm): Crystal grain sizes (Crystallite sizes,X-sizes) of single particles were measured using an X-ray diffractionanalyzer (Bruker AXS D4-Endeavor XRD) for measuring XRD (X-RayDiffraction) with a Cu Kα X-ray (X-rα).

3) Doping amount and metal amount in surface-coated metal compound: Theamount of the doped metal and/or the amount of the metal in thesurface-coated metal compound were/was measuring using an inductivelycoupled plasma spectrometer (ICP).

4) Evaluation of high-temperature characteristics of a 4.35 V full cell.

Each of the positive electrode active materials prepared in thefollowing examples and comparative examples, a carbon black conductivematerial, and a PVdF binder were mixed at a weight ratio of 96:2:2 in anN-methyl pyrrolidone solvent to prepare a slurry for forming a positiveelectrode (viscosity: 5000 mPa-s), and the slurry was applied onto analuminum collector having a thickness of 20 μm, dried, and rolled tofabricate a positive electrode.

Further, artificial graphite MCMB (mesocarbon microbeads) as a negativeelectrode active material, a carbon black conductive material, a PVDFbinder, and a BM-L301 dispersing agent were mixed at a weight ratio of96.5:0.75:1.25:1.5 in an N-methyl pyrrolidone solvent to prepare aslurry for forming a negative electrode, and the slurry was applied ontoa copper collector to fabricate a negative electrode.

A porous polyethylene separator was interposed between the positiveelectrode and the negative electrode fabricated as above to fabricate anelectrode assembly, the electrode assembly was placed inside a case, andthen an electrolyte was injected into the case to fabricate a lithiumsecondary battery. In this regard, the electrolyte was prepared bydissolving 1.15 M of lithium hexafluorophosphate (LiPF₆) in an organicsolvent consisting of ethylene carbonate/dimethyl carbonate/ethyl methylcarbonate (a mixture volume ratio of EC/DMC/EMC=3/4/3).

High-temperature lifetime retention rate (%): The lithium secondarybatteries thus fabricated were charged at 45° C. at constantcurrent/constant voltage (CC/CV) of 0.7 C to 4.35 V/38 mA, and thendischarged at constant current (CC) of 0.5 C to 2.5 V, and a dischargecapacity was measured. This procedure was repeated for 1 cycle to 100cycles, and the results are shown in FIG. 5 .

Examples 1 to 6 (Over-Calcination Temperature: 990° C.) Example 1

94.91702 g of a Ni_(0.5)Co_(0.3)Mn_(0.2)(OH)₂ precursor having aparticle size D50 of 4.2 μm, 38.86741 g of LiOH as a lithium rawmaterial, and 0.444807 g of ZrO₂, 0.010392 g of MgO, and 0.02594 g ofTiO₂ as raw materials of element M were dry-mixed, and thenover-calcined at 990° C. to prepare a LiNi_(0.5)Co_(0.3)Mn_(0.2)O₂positive electrode active material which was doped with 4000 ppm ofZr/Mg/Ti (3500 ppm of Zr, 250 ppm of Mg, and 250 ppm of Ti) with respectto the total weight of the positive electrode active material.

Example 2

A positive electrode active material doped with Zr/Mg/Ti was prepared inthe same manner as in Example 1, except that a precursor of 5.2 μm wasover-calcined at 990° C., and 4250 ppm of Zr/Mg/Ti (3500 ppm of Zr, 250ppm of Mg, and 500 ppm of Ti) was doped.

Example 3

LiNi_(0.5)Co_(0.3)Mn_(0.2)O₂ doped with 4250 ppm of Zr/Mg/Ti of Example2, 0.026291 g of Al₂O₃ as a raw material of element M′, and 0.028722 gof B₂O₃ were mixed and calcined at 990° C. to prepare a doped positiveelectrode active material which was further coated with 900 ppm of Al/B(500 ppm of Al and 400 ppm of B).

Example 4

A positive electrode active material doped with Zr was prepared in thesame manner as in Example 1, except that a precursor of 5.2 μm wasover-calcined at 990° C., and 3500 ppm of Zr was doped by using 0.444807g of ZrO₂ as a raw material of element M.

Example 5

A positive electrode active material doped with Zr was prepared in thesame manner as in Example 1, except that a precursor of 5.2 μm wasover-calcined at 990° C., and 5500 ppm of Zr was doped by using 0.698983g of ZrO₂ as a raw material of element M.

Example 6

LiNi_(0.5)Co_(0.3)Mn_(0.2)O₂ doped with 4250 ppm of Zr/Mg/Ti of Example2 and 0.078872 g of Al₂O₃ as a raw material of element M′ were mixed andcalcined at 990° C. to prepare a doped positive electrode activematerial which was further coated with 1500 ppm of Al.

Examples 7 to 12 (Over-Calcination Temperature: 1010° C.) Example 7

A LiNi_(0.5)Co_(0.3)Mn_(0.2)O₂ positive electrode active material whichwas doped with 4000 ppm of Zr/Mg/Ti (3500 ppm of Zr, 250 ppm of Mg, and250 ppm of Ti) with respect to the total weight of the positiveelectrode active material was prepared in the same manner as in Example1, except that over-calcination was performed at a temperature of 1010°C. instead of 990° C. as in Example 1.

Example 8

A positive electrode active material doped with Zr/Mg/Ti was prepared inthe same manner as in Example 1, except that over-calcination wasperformed at a temperature of 1010° C. instead of 990° C. as in Example2, and 4250 ppm of Zr/Mg/Ti (3500 ppm of Zr, 250 ppm of Mg, and 500 ppmof Ti) was doped in the same manner as in Example 2.

Example 9

LiNi_(0.5)Co_(0.3)Mn_(0.2)O₂ doped with 4250 ppm of Zr/Mg/Ti wasprepared in the same manner as in Example 3, except thatover-calcination was performed at a temperature of 1010° C. instead of990° C. as in Example 3. Thereafter, the doped positive electrode activematerial was further coated with 900 ppm of Al/B (500 ppm of Al and 400ppm of B) in the same manner as in Example 3.

Example 10

A positive electrode active material doped with 3500 ppm of Zr wasprepared in the same manner as in Example 4, except thatover-calcination was performed at a temperature of 1010° C. instead of990° C. as in Example 4.

Example 11

A positive electrode active material doped with 5500 ppm of Zr wasprepared in the same manner as in Example 5, except thatover-calcination was performed at a temperature of 1010° C. instead of990° C. as in Example 5.

Example 12

LiNi_(0.5)Co_(0.3)Mn_(0.2)O₂ doped with 4250 ppm of Zr/Mg/Ti wasprepared in the same manner as in Example 6, except thatover-calcination was performed at a temperature of 1010° C. instead of990° C. as in Example 6. Thereafter, the doped positive electrode activematerial was further coated with 1500 ppm of Al in the same manner as inExample 6.

Comparative Example 1

A positive electrode active material doped with 4250 ppm of Zr/Mg/Ti wasprepared in the same manner as in Example 2, except that calcination wasperformed at a temperature of 920° C.

Comparative Example 2

A positive electrode active material doped with 4250 ppm of Zr/Mg/Ti wasprepared in the same manner as in Example 2, except that calcination wasperformed at a temperature of 950° C.

Comparative Example 3

A positive electrode active material doped with 1500 ppm of Zr wasprepared in the same manner as in Example 4, except that 1500 ppm of Zrwas doped.

Comparative Example 4

A positive electrode active material doped with 7500 ppm of Zr wasprepared in the same manner as in Example 4, except that 7500 ppm of Zrwas doped.

Comparative Example 5

A LiNi_(0.5)Co_(0.3)Mn_(0.2)O₂ positive electrode active material wasprepared in the same manner as in Example 4, except that no Zr-dopingtreatment was performed.

Comparative Example 6

A positive electrode active material doped with 4250 ppm of Zr/Mg/Ti wasprepared in the same manner as in Example 2, except that a precursor of11 μm was calcined at a temperature of 920° C.

Comparative Example 7

A positive electrode active material doped with 1500 ppm of Zr wasprepared in the same manner as in Example 4, except that a precursor of12 μm was over-calcined at a temperature of 960° C., and 1500 ppm of Zrwas doped.

Experimental Example 1 (Confirmation of Crystal Grain Size by Cu Kα XRDAnalysis)

Each of the active materials prepared in Examples 2, 4, 6, 8, 10, and 12was observed and analyzed using a scanning electron microscope (SEM),etc., and the results are shown in FIGS. 1A (Example 2), 1B (Example 4),1C (Example 6), 1D (Example 8), 1E (Example 10), and 1F (Example 12).Further, crystal grain sizes of the positive electrode active materialsof Examples 1 to 8 and 10 to 12, and Comparative Examples 1 to 7, wereconfirmed using an X-ray diffraction analyzer (Bruker AXS D4-EndeavorXRD), and the results are shown in the following Table 1.

In detail, the crystal grain sizes of the positive electrode activematerials were calculated by performing X-ray diffraction analysis usinga Cu source (Cu Kα), obtaining a full width at half maximum (FWHM) fromthe analysis results, and then putting the full width at half maximuminto the following equation.τ=(K·λ)/(β−cos θ)  [Equation]

The above equation is widely known as the Scherrer equation, wherein Kis a shape factor, usually 0.9 without a unit, λ is X-ray wavelength, βis a full width at half maximum of a peak having a maximum intensity,and θ is an angle of peak maximum. The corresponding values are put intothe equation, thereby calculating the crystal grain size τ.

TABLE 1 Total amount of coating Crystal grain and doping (ppm) size (nm)Example 1 4000 264 Example 2 4250 233 Example 3 5150 207 Example 4 3500225 Example 5 5500 233 Example 6 5750 229 Example 7 4000 181 Example 84250 194 Example 10 3500 194 Example 11 5500 195 Example 12 5750 206Comparative Example 1 4250 151 Comparative Example 2 4250 178Comparative Example 3 1500 215 Comparative Example 4 7500 239Comparative Example 5 0 232 Comparative Example 6 4250 154 ComparativeExample 7 1500 199

Referring to FIGS. 1A to 1F, it was confirmed that the active materialsprepared according to the present invention were prepared in the form ofsingle particles having a particle size of 3.5 μm to 10 μm.

Further, referring to Table 1, it was confirmed that when a precursorhaving an average particle size of 7 μm or less was used, andover-calcination was performed at 960° C. or higher (Examples 1 to 12),the crystal grain size as measured by Cu Kα X-ray (X-rα) was relativelylarge in the range of 180 nm to 300 nm. In contrast, it was confirmedthat when the calcination temperature was lower than 960° C.(Comparative Examples 1, 2, and 6), the crystal grain size was less than180 nm, irrespective of the size of the precursor, indicating not beingover-calcined.

Meanwhile, at the same compositions, it is common that as thecalcination temperature increases, the size of the single particle andthe size of the crystal grain in the single particle increase. However,according to results measured by Cu Kα X-ray (X-rα), the sizes of thecrystal grains over-calcined at 960° C. (Examples 1 to 6), as measuredby Cu Kα X-ray (X-rα), were larger than those of the crystal grainsover-calcined at 1010° C. (Examples 7 to 12), when their compositionswere the same as each other. This is because the patterns resulting fromover-calcination at 1010° C. (Examples 7 to 12) are related to anaggregation degree of single particles according to the calcinationtemperature and a measuring device, rather than being out of the generaltendency.

When single particles are over-calcined at 990° C. (Examples 1 to 6),relatively small single particles may aggregate together and look likesecondary particles. That is, it is understood that XRD recognizes thesecondary particle-like look, and recognizes boundaries between thesingle particles as grain boundaries to detect the single particle sizeas the crystal grain size.

Unlike this, when single particles are over-calcined at 1010° C.(Examples 7 to 12), relatively large single particles may be separatelyobserved while maintaining a distance from each other. Therefore, it isunderstood that XRD recognizes individual single particles, andcorrectly recognizes the crystal grain size in the single particles.

In other words, those over-calcined at 1010° C. (Examples 7 to 12)showed the larger crystal grains than those over-calcined at 960° C.(Examples 1 to 6), as measured by Cu Kα X-ray (X-rα), which isattributed to limitations of the measuring device.

However, all of those over-calcined at 960° C. (Examples 1 to 6) andthose over-calcined at 1010° C. (Examples 7 to 12) showed practicalsizes of the crystal grains within the error range (±1%) of the valuemeasured by Cu Kα X-ray (X-rα), that is, within the range of 180 nm to300 nm.

Experimental Example 2 (Evaluation of Crystal Structure)

Example 2 and Comparative Example 1 were observed using TEM-ASTAR andHR-TEM equipment. Specifically, an HR-TEM image of Example 2 is shown inFIG. 2A, a TEM-ASTAR image of the surface thereof is shown in FIGS. 2Band 2C, and a TEM-ASTAR image of the central part thereof is shown inFIG. 2D. Further, an HR-TEM image of Comparative Example 1 is shown inFIG. 2E, a TEM-ASTAR image of the surface thereof is shown in FIG. 2F,and a TEM-ASTAR image of the central part thereof is shown in FIG. 2G.

Referring to FIGS. 2A to 2G, the crystal structure of the surface partmay differ depending on the calcination temperature of the precursor,even though the compositions are the same. Specifically, ComparativeExample 1 which was not over-calcined showed uniform formation of alayered crystal lattice structure from the surface thereof to the centerthereof.

In contrast, Example 2 which was over-calcined showed different crystalstructures in the surface thereof and the center thereof. Morespecifically, the over-calcined Example 2 had a single particle D50 of6.8 μm, and had a rock salt structure, a spinel structure, or a mixedstructure thereof from the surface part thereof to a depth of 25 nm, andthe central part thereof is a layered structure.

Experimental Example 2 (Evaluation of Gas Generation)

The positive electrode active materials according to the examples andcomparative examples were measured for gas generation and the resultsare shown in the following Tables 2 and 3, and FIGS. 2A and 2B.

Specifically, a method of measuring the gas generation is as follows.

Two NCM electrodes (loading amount per electrode area: 1000 mg/cm²)charged at a 4.35 V voltage (based on a full cell) and two separatorswere placed on the bottom plate of a coin cell, and fixed with a gasket,and 80 μl of an electrolyte (DEC REF. (EC/PC/DEC=3/1/6, VC/PS=0.5/1 Wt%)) was injected under vacuum twice. Each surface was vacuum-sealed at athickness of 0.5 cm using an aluminum pouch (Al pouch) of 6.5*10 cm.Here, the vacuum sealing means sealing under monocell vacuum sealingconditions of 95 kPa/93 kPa. Thereafter, each battery was stored in aconvection oven at 60° C. for 2 weeks, and then gas generation in thebattery was measured.

TABLE 2 Kind of gas Example 1 Example 2 Example 3 Example 4 Example 5Example 6 Gas H₂ 0 0 0 0 0 0 generation CO 586.4 524.8 535.6 597.2222222402.7 262.2504537 (μL) CO₂ 899.7 891.3 871.1 805.5555556 639.2412.8856624 CH₄ 0 0 0 0 0 0 C₂H₂ 0 0 0 0 0 0 C₂H₄ 101.2 118.3 100.881.1965812 117.8 0 C₂H₆ 0.0 0.0 0.0 0 0.0 0 C₃H₆ 49.8 50.8 46.825.64102564 40.7 26.31578947 C₃H₈ 0.0 0.0 0.0 0 0.0 0 Σ (9 kinds 1638.61582.7 1556.8 1509.6 1203.7 701.4519056 of gases)

TABLE 3 Kind of gas Example 7 Example 8 Example 9 Example 10 Example 11Example 12 Gas H₂ 0 0 0 0 0 0 generation CO 597.2222222 295.4545455504.8638132 283.7620579 486.0486049 95.54140127 (μL) CO₂ 805.5555556795.4545455 885.2140078 1020.900322 612.9612961 286.6242038 CH₄ 0 0 0 00 0 C₂H₂ 0 0 0 0 0 0 C₂H₄ 81.1965812 215.9090909 97.27626459 138.26366560 222.9299363 C₂H₆ 0 0 0 0 0 0 C₃H₆ 25.64102564 147.7272727 35.992217952.25080386 19.8019802 53.07855626 C₃H₈ 0 0 0 0 0 0 Σ (9 kinds1506.410256 1454.545455 1527.237354 1495.2 1116.111611 658.1740977 ofgases)

Referring to Tables 2 and 3 and FIGS. 3A and 3B, when over-calcinationwas performed at 960° C. or higher (Examples 1 to 12), gas generationwas remarkably reduced.

Those over-calcined at 1010° C. (Examples 7 to 12) showed less gasgeneration than those over-calcined at 990° C. (Examples 1 to 6), whichis attributed to the reduced amount of residual lithium and reduction ofcracks inside the over-calcined particles and interface of the particlesurface. Here, the cracks inside the particles and interface of theparticle surface are regions where side reactions with the electrolytemay occur. Due to reduction of the regions where side reactions mayoccur, gas generation was remarkably reduced.

Experimental Example 3 (Evaluation of High-Temperature OperatingCharacteristics of Battery)

Each of the positive electrode active materials according to theexamples and comparative examples was used to prepare a slurry forforming a positive electrode and a lithium secondary battery, and thentheir high-temperature lifetime characteristics were evaluated and theresults are shown in the following FIGS. 4A and 4B.

Referring to FIGS. 4A and 4B, the active materials of Examples 1 to 12having all technical compositions of over-calcination of precursorparticles and limited doping (and coating) contents showed excellenthigh-voltage and high-temperature lifetime characteristics. In contrast,Comparative Examples 1 and 2 not satisfying the over-calcinationconditions showed capacity retention of 85% or less at 100 cycles, andthe slopes gradually increased. Thus, it is expected that the capacityretention will more rapidly decrease after 100 cycles. ComparativeExamples 3 and 4 of which coating and doping contents were out of theconditions showed that capacity retention decreased and the slopesgradually decreased with cycling. Thus, it is expected that the capacityretention will further decrease after 100 cycles. Comparative Example 5in which coating and doping were not performed showed excellent initialcapacity retention, but showed a rapid decrease after 70 cycles.Comparative Example 6 in which over-calcination did not occur and thesize of the precursor was large showed a rapid decrease in initialcapacity retention. Comparative Example 7 in which over-calcinationoccurred but the precursor size and the coating/doping contents did notsatisfy the conditions of the present invention showed a rapid decreasein initial capacity retention and deterioration in lifetimecharacteristics with cycling.

In particular, those over-calcined at 1010° C. (Examples 7 to 12) showedexcellent capacity retention of the battery compared to thoseover-calcined at 990° C. (Examples 1 to 6), which may be associated withgas generation, as confirmed in Experimental Example 2. Specifically,those over-calcined at 1010° C. (Examples 7 to 12) showing low Corelease may improve electrochemical properties of the battery, ascompared with those over-calcined at 990° C. (Examples 1 to 6).

However, all of those over-calcined at 990° C. (Examples 1 to 6) andthose over-calcined at 1010° C. (Examples 7 to 12) showed remarkableimprovement in the capacity retention, as compared with those of thecomparative examples.

While preferred exemplary embodiments of the present invention have beendescribed in detail, the scope of the present invention is not limitedto the foregoing embodiment, and it will be appreciated by those skilledin the art that various modifications and improvements using the basicconcept of the present invention defined in the appended claims are alsoincluded in the scope of the present invention.

The invention claimed is:
 1. A positive electrode active material for alithium secondary battery, the positive electrode active materialcomprising a nickel-based lithium composite metal oxide single particle,wherein the single particle includes a plurality of crystal grains, eachcrystal grain having a size of 180 nm to 300 nm as measured by X-raydiffraction analysis with a Cu Kα X-ray (X-rα), and a metal doped in acrystal lattice of the single particle, wherein a total weight of themetal doped is 2500 to 6000 ppm, wherein the metal is one or more metalsselected from the group consisting of Al, Ti, Mg, Zr, W, Y, Sr, Co, F,Si, Na, Cu, Fe, Ca, S, and B.
 2. A positive electrode active materialfor a lithium secondary battery, the positive electrode active materialcomprising: a nickel-based lithium composite metal oxide singleparticle, wherein the single particle includes a plurality of crystalgrains, each crystal grain having a size of 180 nm to 300 nm as measuredby Cu Kα X-ray (X-rα), and a metal doped in a crystal lattice of thesingle particle, wherein the metal doped in a crystal lattice of thesingle particle is one or more metals selected from the group consistingof Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S, and B; and ametal (compound coated on a surface of the single particle, wherein themetal compound is one or more metals selected from the group consistingof Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S, and B,wherein a total content of the metal doped in the crystal lattice of thesingle particle and the metal in the metal compound coated on thesurface thereof is 2500 ppm to 6000 ppm.
 3. The positive electrodeactive material for a lithium secondary battery of claim 1, wherein thesingle particle includes, in the crystal lattice, a surface part havinga rock salt structure, a spinel structure, or a mixed structure thereoffrom a surface of the single particle to a depth of 0.13% to 5.26% of aradius of the lithium composite metal oxide single particle, and acentral part having a layered structure from an interface with thesurface part thereof to the center part of the single particle.
 4. Thepositive electrode active material for a lithium secondary battery ofclaim 1, wherein the single particle has an average particle size (D50)of 3.5 μm or more to 8 μm or less.
 5. The positive electrode activematerial for a lithium secondary battery of claim 1, wherein thenickel-based lithium composite metal oxide is represented by ChemicalFormula 1:Li_(a)(Ni_(x)Mn_(y)Co_(z))O_(2+b)  (1) wherein, 0.95≤a≤1.2, 0≤b≤0.02,0<x<0.6, 0≤y≤0.4, 0≤z<0, and x+y+z=1.
 6. The positive electrode activematerial for a lithium secondary battery of claim 5, wherein in ChemicalFormula 1, a=1, 0≤b≤0.02, 0.4≤x<0.6, 0.1≤y<0.4, 0.1≤z<0.4, and x+y+z=1.7. The positive electrode active material for a lithium secondarybattery of claim 5, wherein Chemical Formula 1 isLiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ or LiNi_(0.5)Co_(0.3)Mn_(0.2)O₂.
 8. Thepositive electrode active material for a lithium secondary battery ofclaim 1, wherein the metal is Ti, Mg, or Zr.
 9. The positive electrodeactive material for a lithium secondary battery of claim 1, wherein themetal is Zr.
 10. A method of preparing a positive electrode activematerial for a lithium secondary battery, comprising: preparing a firstmixture including a nickel-based lithium composite metal hydroxideparticle having an average particle size (D50) of 8 μm or less, alithium raw material, and a metal compound, wherein the metal compoundis one or more selected from the group consisting of Al, Ti, Mg, Zr, W,Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S, and B; and calcining the firstmixture at a temperature of 960° C. or higher, wherein a content of themetal compound in a total weight of the first mixture is 2500 ppm to6000 ppm.
 11. A method of preparing a positive electrode active materialfor a lithium secondary battery, comprising: preparing a first mixtureincluding a nickel-based lithium composite metal hydroxide particlehaving an average particle size (D50) of 8 μm or less, a lithium rawmaterial, and a first metal compound, wherein the first metal compoundis one or more metals selected from the group consisting of Al, Ti, Mg,Zr, W, Y, Sr, Co, F, Si, Na, Cu, Fe, Ca, S, and B; calcining the firstmixture at a temperature of 960° C. or higher to obtain a calcinedproduct of the first mixture; preparing a second mixture including thecalcined product of the first mixture, and a second metal compound,wherein the second metal compound is one or more metals selected fromthe group consisting of Al, Ti, Mg, Zr, W, Y, Sr, Co, F, Si, Na, Cu, Fe,Ca, S, and B; and calcining the second mixture at a temperature of 350°C. to 800° C., wherein a total content of the first metal compound andthe second metal compound in a total weight of the first mixture and thesecond mixture is 2500 ppm to 6000 ppm.
 12. The method of claim 10,wherein the nickel-based lithium composite metal hydroxide particle isrepresented by Chemical Formula 2:(Ni_(x)Mn_(y)Co_(z))OH_(2+b)  (2) wherein, 0.95≤a≤1.2, 0≤b≤0.02,0<x<0.6, 0≤y≤0.4, 0≤z<1, and x+y+z+=1.
 13. The method of claim 10,wherein in the calcining the first mixture at a temperature of 960° C.or higher, a nickel-based lithium composite metal oxide single particleincluding a plurality of crystal grains is synthesized, and at the sametime, the metal of the metal compound is doped in a crystal lattice ofthe nickel-based lithium composite metal oxide single particle.
 14. Themethod of claim 11, wherein the second metal of the second metalcompound is different from the first metal of the first metal compound.15. A positive electrode for a lithium secondary battery, the positiveelectrode comprising the positive electrode active material of claim 1.16. A lithium secondary battery comprising the positive electrode ofclaim
 15. 17. The method of claim 10, wherein the calcining is at atemperature from 960° C. to 1100° C.
 18. The method of claim 10, whereinthe nickel-based lithium composite metal hydroxide particle has a D50 of4 μm or more to 8 μm or less.
 19. The method of claim 11, wherein thecalcining is at a temperature from 960° C. to 1100° C.
 20. The method ofclaim 11, wherein the nickel-based lithium composite metal hydroxideparticle has a D50 of 4 μm or more to 8 μm or less.